1. Technical Field of the Invention
This invention pertains generally to methods and systems for the determination of an ion concentration in a liquid sample. More specifically, the invention pertains to a method and system for the determination of bromide ion concentrations in aqueous samples.
2. Description of the Related Art
While bromides are generally considered to be non-toxic salt compounds, bromide contamination of water is of great concern due to the resultant brominated organics, specifically brominated trihalomethanes, which form during chlorination of drinking water. Studies have shown a link between ingestion of brominated trihalomethanes and several types of cancer and birth defects.
Bromide is most often found in nature in the form of salts with sodium, potassium, and other ions, and occurs in varying amounts in ground and surface waters. For example, bromide concentrations in seawater are generally in the range of 65 mg/l to well over 80 mg/l, while concentrations in fresh water typically range from trace amounts to about 0.5 mg/l (Bromide in the Natural Environment: Occurrence and Toxicity, M. Flury & A. Papritz, Journal of Environmental Quality 22(4):747-758, October-December 1993; Bromide in Drinking water, Background document for development of WHO Guidelines for Drinking-water Quality, World Health Organization 2009). Fresh water sources located near coastal regions may have much higher concentrations as a result of seawater intrusion and sea-spay effected precipitation. Further, the bromide content of ground waters and stream base-flows may also be affected by connate water (e.g., water trapped in geological formations).
One additional source of elevated bromide levels in freshwater supplies is the hydraulic fracturing of Marcellus shale formations used in the production of natural gas. While most geological formations have little bromide, limestone and shale are rich in this ion. After hydraulic fracture is complete, the internal pressure of the geologic formation causes water injected during fracture to rise to the surface where it is generally recovered and stored in tanks or pits prior to disposal or recycling. This recovered water, commonly referred to as produced water or flow-back water, carries with it numerous chemicals added during the hydraulic fracturing process in addition to salts (chlorides, bromides, and sulfides of calcium, magnesium, and sodium), metals (barium, manganese, iron, and strontium, among others), bacteria and hydrocarbons leached from the geologic formation. As such, typical flow-back water may contain contamination levels of salts, metals and dissolved solids (total organic and inorganic dissolved solids) that are as much as several thousand fold over the EPA's maximum allowable level.
TABLE 1Flowback WaterEPA's MaximumChemistryContaminant LevelMultiples of(mg/L)(mg/L)ContaminationBromide4450.00005 as ethylenebromide0.01 as bromate>30,0000.08 as totaltrihalomethanesChloride41,850250167TDS*67,300500135HardnessUp to 55,000—Extremelyhard*Total Dissolved Solids
One clear example of bromide contamination was found in the Monongahela River in western Pennsylvania. A study conducted by Carnegie Mellon University found that bromide levels rose in 2010 and have remained elevated. Whether this is due to direct discharge of flow-back water into the river, or contamination of local tributaries due to poor or improper handling is not clear. Consequently, communities in close proximity to such operations are greatly concerned over whether or not the water recycling to and from surface containment tanks is carried out correctly. Table 1 shows analytical data on hydraulic fracture flow-back water sampled from the Marcellus Shale geological area (Sampling and Analysis of Water Streams Associated with the Development of Marcellus Shale Gas, Prepared for “Marcellus Shale Coalition” by Thomas Hayes, Dec. 31, 2009). Of significance is that the bromide ion was found at average concentrations of 445 mg/L, approximately 30,000 times greater than that of normal surface water. Improper handling of such water would easily lead to contamination of freshwater sources.
Table 1 also shows the extremely high hardness levels of the flow-back water, which are indicative of high concentrations of total dissolved salts, such as bromide, chloride, magnesium and manganese. While low levels of certain of these salts in aquifers are crucial for the growth and development of wildlife and fish in the Marcellus geologic area, high concentrations can be extremely deleterious. For example, there are eleven public water treatment intakes on the Monongahela River, supplying approximately 350,000 customers. The bromide contamination recently found in this water source may react with the disinfectants used at these eleven public water treatment plants to form brominated trihalomethanes. The greater the contaminations level in the river water, the more brominated trihalomethanes in the treated water, and studies have shown a link between ingestion of trihalomethanes and several types of cancer and birth defects.
A simple and low cost, fast response sensor for bromide ion would be desirable for identifying contamination of ground and surface waters. Ideally, a sensor would be capable of establishing a concentration gradient leading from dilute concentrations as low as single digit parts-per-billion up to the sources of contamination at hundreds of mg/L. Thus, such a sensor would require sensitivity to levels of bromide ion spanning several orders of magnitude.
Other potential applications of such a method and sensor include monitoring of bromide ion in solution mined sodium chloride brine, as is used as feedstock in the chlor-alkali process to produce chlorine and sodium hydroxide (caustic soda). Bromide in brine would become a bromine contaminant in chlorine, which is of concern in many of chlorine's downstream uses.
Bromide concentrations have historically been determined spectrophotometrically by the bromination of a chemical dye after oxidation of bromide. Prior art methods for the determination of bromide concentrations have used high heats, strong acids, and compounds such as carbon tetrachloride or concentrated hydrogen peroxide, all of which pose ecological and health hazards. Further, the processes have involved distillation, extensive separations, and/or centrifugation steps. As such, none have been amenable to rapid detection of bromide ion concentrations. And none have been portable so that they may be used in the field.
Thus, what is needed is a rapid and low cost method and system for the detection of bromide ion over a broad concentration range.